The shoulder joint is found where the head of the humerus mates with a shallow depression in the scapula. The movement of the humerus relative to the scapula is controlled by a number of muscles including: the deltoid, the supraspinatus, the infraspinatus, the subscapularis, and the teres minor. Various patterns of activating these muscles create various rotational moments because the shoulder joint has no fixed axis. The tendons linking the supraspinatus, the infraspinatus, the subscapularis, and the teres minor to the humerus are typically referred to as the rotator cuff tendons. Some studies suggest that 85% of people over the age of 65 have some degree of rotator cuff damage. This damage may include thinning, fraying, and/or tearing of the rotator cuff tendons. Various factors may contribute to this rotator cuff damage. These factors include aging, overuse of the shoulder, and wearing of the tendons. Wearing of the tendons can occur, for example, when the rotator cuff tendons rub against a bone (e.g., the acromium of the scapula).